Now some of the "new physiognomists" are resurrecting an old claim: that you can gauge a man's penchant for aggression by the cut of his jib. Last fall University of California-Santa Barbara psychologist Aaron Sell reported that college students could accurately estimate the upper body strength of unfamiliar men after viewing their faces alone. (The men's necks were obscured.) The students did equally well with fellow undergraduates and men from South American indigenous groups—all of whom had had their strength measured using gym equipment. Interestingly, the toughest-looking undergrads also reported getting in the most fights. Another study by Sell suggests that such formidable men are more prone to use violence—or advocate military action—to resolve conflicts.

Many animals employ similar systems. Male orangutans grow fatty cheek pads that reflect group status. Lions with long, dark manes tend to rule the pride. From an evolutionary perspective, these advertisements may be a convenient way of saying, "Hey bro—btw, I can kick your ass" without having to go through the risk of combat.

So which features might hint at belligerence? Sell suspects the brow ridge and jaw, two structures that are shaped by testosterone in puberty. (High testosterone has been linked with masculine looks as well as with aggression.) Other scientists propose a different measure: the width-to-height ratio of the face, as measured from cheek to cheek and lip to brow. Last year, a team of Canadian psychologists showed that men with wider faces (think Ernie) score higher in lab tests of aggression than slender-faced men (think Bert). They also found that wide-faced hockey players rack up more penalty minutes. Now, two studies in Psychological Science—one from August and another forthcoming—reinforce the notion that stout-faced men appear tougher and are more likely to behave in aggressive and untrustworthy ways.